1. Field of the Invention
This invention relates to a process for removing undesirable substances, e.g., water, oil and air, entrained within various insulating materials as well as other components of non-operating electrical devices.
More specifically, the present invention relates to removing substances that degrade and otherwise adversely affect the functioning of the components, e.g., transformer cores, of electrical devices, particularly of the "oil-filled" type. The present invention further relates to retrofilling oil-filled electrical devices, particularly transformers.
2. State of the Art
Among the more commonly used commercial and industrial electrical devices are the "oil-filled" type. The oil acts as an insulator and at the same time as a cooling fluid when used in such devices as transformers, power capacitors, various cables, switches, circuit breakers and the like. In the recent past, the liquids which have found the widest use in these various "oil-filled" electrical devices are silicone oils, which have been limited in their use due to their expense, and polychlorinated biphenyls and diphenyls (PCB's). These and similar materials have been disclosed in the art, for example, in U.S. Pat. Nos. 1,931,373; 2,139,948 and 4,177,156. While PCB's and the like have been found to be functionally advantageous, the industry has moved away from using these materials due to their toxicity and environmental impact. Substitute oils such as halogenated aliphatic hydrocarbons are now coming into use replacing PCB's and like materials. Such halogenated aliphatic hydrocarbons are disclosed in U.S. Pat. Nos. 1,953,216 to Elsey; 1,966,901 to McMahon; and 2,019,338 to Clark. Furthermore, various fluorinated and chlorinated aliphatic hydrocarbons have found use in a number of applications for electrical devices. For example, U.S. Pat. No. 3,733,218 discloses azeotropic solvent mixtures of trichloroethylene/isobutanol, perchloroethylene/cyclopentanol, perchloroethylene/ethylene glycol monoethyl ether and perchloroethylene/N-amyl alcohol used to clean rosin-based solder circuit boards. U.S. Pat. No. 3,957,531 teaches similar azeotropic solvent mixtures for cleaning circuit boards by immersion in a tank containing such a solvent mixture. It is disclosed in U.S. Pat. No. 4,054,036 that a constant boiling mixture of 1,1,2-trichlorotrifluoroethane and cis-1,1,2,2-tetrafluorocyclobutane are useful as refrigerants, heat transfer media, gaseous dielectrics and the like. It is also disclosed in U.S. Pat. No. 4,276,530 that a fluorocarbon liquid and perchloroethylene are useful as a water collector for a vapor-cooled electrical apparatus which is hermetically sealed to the atmosphere where the disclosed process is applicable solely for an operating electrical apparatus.
The replacement of PCB's as the oils used in the previously mentioned electrical devices has resulted in either the need for new "oil-filled" electrical devices coming on the market replacing those in existence or the retrofilling of those presently in use. The term "retrofilling," as used herein, is to be understood to mean the removal of the dielectric fluid material contained in an electrical device which generally has been in use, optionally purging the emptied electrical device followed by the refilling of the electrical device with the same or different dielectric fluid material. Problems, however, have been encountered with the installation of these new electrical devices, in particular transformers. During the preparation of transformer cores as well as the internal components of the other mentioned electrical devices, atmospheric water is absorbed by the insulator material which is conventionally kraft paper. Also air is entrapped in the core during the winding procedure. As is readily recognized by one skilled in the art, the presence of even small amounts of water and oxygen will create problems in transformer cores as well as the internal components of capacitors or switches. These substances will degrade the materials of the particular component potentially creating "shorts" which in turn will cause more heat to be generated by the component which further accelerates and aggravates the problem. Furthermore, materials finding use as substitute oils (such as halogenated aliphatic hydrocarbons including perchloroethylene and trichloroethylene), while quite stable in the absence of oxygen and light, will autooxidize quite rapidly when exposed to oxygen particularly in the presence of ultraviolet radiation to degrade to various undesirable corrosive products as acetyl chlorides, carboxylic acids, hydrogen chloride and phosgene. Therefore, it has been quite critical during the installation, refilling or maintenance of the electrical device to insure that substantially all degradative or corrosive substances have been removed from the internal components prior to putting the device into operation. In the past, in the case with transformers, this drying or purging procedure has been accomplished by allowing the transformer core to heat up to relatively high temperatures (250.degree. C. or more) under greatly reduced pressure (1 mmHg). This method, however, is not all that satisfactory since it requires 5 to 6 hours or more and it is obviously energy intensive. Also, this method has not always been as effective as desired since residual atmospheric water and air is reabsorbed by the core material.
To retrofill an electrical device, particularly a transformer which contained PCB's, it is necessary to reduce the PCB concentration level to below 500 ppm and most preferably below 50 ppm. There is no known method which can achieve these PCB levels economically and, thus, it has been necessary to dispose of the electrical device. Disposing of an electrical device can represent a major economic loss particularly when the device is a transformer.
Therefore, there is a need for a method for removing undesirable and degradative substances from the internal components of "oil-filled" electrical devices, particularly transformers which method is relatively fast, economical, less energy intensive and most importantly highly effective in removing substantially all of the undesirable substances while not allowing them to re-enter the system.